In order to achieve compatibility between durability and transport efficiency at a high level, the conventional technology, for example, (1) configured tread rubber to have a laminated structure of cap rubber and base rubber, the cap rubber having excellent wear resistance, and the base rubber being low heat generation-type rubber, so as to decrease an amount of heat generation of the tread rubber, (2) decreased strain between belt layers, and (3) adopted rubber having excellent fracture resistance in the belt layers.
However, according to the conventional technology, in the event of fracture of rubber occurring at a side edge of a belt composed of the plurality of belt layers, the fracture is likely to progress to the base rubber made of the low heat generation-type rubber that is beyond the belt layers. The likelihood of progression of fracture has been a cause that acts against further improvement in durability of the tire, more directly, the tread rubber.
That is to say, in rubber, fracture resistance is considered to be incompatible with low heat generation. However, in a radial tire type of the kind that includes a plurality of steel belt layers, for example, due to discrepancy between deformation resulting from a flow of rubber during ground-contact and deformation of the entire belt layers, shearing deformation occurs at the side edge of an outermost belt layer and/or a widest-width belt layer mainly in a tread circumferential direction and in a tread width direction. Accordingly, in the above radial tire, as FIG. 6 illustrates a partial enlarged sectional view of the tread portion in the width direction, for example, fracture c forming an angle of approximately 20°-30° with respect to a tangent line to of a surface of the outermost belt layer progresses from the side edge position of the outermost belt layer to a tire equatorial plane E within low heat generation-type base rubber br. As a result, in the above radial tire, durability of the tread rubber TR is deteriorated at a relatively early stage.